Metal workpieces are welded by directing a laser beam of suitable energy at a selected weld location on the surface of the metal. Some of the energy of the laser radiation is absorbed by the metal, melting the material at the region of incidence to a depth suitable to join the metal layers. After the laser beam is shut off or moved away along a weld path in the workpiece, the molten metal quickly dissipates heat to the surrounding unheated metal and re-solidifies to form a weld nugget between the layers to be joined. High power carbon dioxide (CO2) gas lasers and neodymium: YAG solid-state lasers, for example, are used in material processing applications including machining, heat treating and welding.
An advantage of laser welding is that a beam of coherent radiation can be focused to quickly, but momentarily, form a deep narrow keyhole of molten metal for the weld. But a disadvantage is that the weld nugget formed in this rapid manner often exhibits embrittlement or hot cracking along the weld joint, especially in light metal alloys with relatively high thermal conductivity. Efforts are made to control the energy level and/or the path of the laser to produce a puddle of weld metal of suitable shape to reduce embrittlement of the weld nugget, but in many laser welding applications porosity and/or hot cracking of the weld nugget remains as a problem.
It is an object of this invention to provide a method of welding metal workpieces by combining the energies of two laser beams of different wavelengths so as to reduce shrinkage stresses and, consequently, hot cracking in the weld nugget and other weld defects. It is a more specific object of this invention to provide an improved method of welding aluminum or magnesium workpieces by combining two laser beams and controlling their respective power levels to produce welds without crack formation in the weld nugget.